This invention relates to an apparatus for forming a trench along the bed of a body of water and more particularly to a machine for forming a trench for burying a pipeline or cable lying along a seabed.
In the energy, telecommunications and other industries, it has been a common practice to lay pipelines, cables and other types of conduits along seabeds for conveying fluids such as oil and gas and communications data from offshore rigs to a mainland and between mainlands. Typically, such conduits are first laid along the seabed and then are buried so as to avoid any damage thereto resulting in a loss of fluid conveyed or disruption of data transmitted therethrough.
In the prior art, there has been developed a number of machines for forming a trench for burying such conduits. Typically, such machines have consisted of a frame adapted to straddle a conduit lying along a seabed, means mounted on a machine frame and engageable with the conduit for propelling the machine along the conduit, means mounted on the machine for ejecting high velocity streams of water to dislodge the seabed along the conduit and thus form a trench into which the conduit is caused to fall as the machine is propelled along the conduit, and often means also mounted on the machine for educting spoils produced by the high velocity streams injected by the machine and thus facilitating the formation of the trench. Examples of such machines are illustrated and described in U.S. Pat. Nos. 3,751,927 to Joseph C. Perot, Jr., 4,087,981 to Robert M. Norman, 4,112,695 to Mike M. Chang et al and 4,117,689 to Charles F. Martin.
The conventional means for propelling such prior art trenching machines along a conduit to be buried has consisted of a set of rollers, one or more of which are adapted to be driven, usually by a hydraulic motor mounted on the machine frame. Such means of propulsion, however, has been found not to be entirely effective in providing uninterrupted and consistent travel of the trenching machine along the conduit. The use of rollers as a propulsion means has been found to be ineffective in providing sufficient traction with the conduit for propelling the machine along the conduit, particularly when the conduit is formed of a material with a smooth surface such as a plastic coating, causing damage to the conduit when the traction force is excessive and fractures the conduit often consisting of a concrete pipeline, and in negotiating projections on the conduits such as anodes commonly provided on underwater pipelines.
In a typical operation for burying a conduit lying along a seabed, there is provided a service barge, a submersible trenching machine adapted to be launched from the service barge and attached to the conduit and an umbilical line interconnecting pumps and compressors provided on the barge and the trenching machine. Operators of such barges and machines commonly are compensated by the linear footage of conduit buried. Interruptions of the trenching operations of such machines are very costly to such operators not only because of the loss of revenue in operation of the trenching machine but also because of the cost of operating the service barge and the equipment on board.
Accordingly, the principal object of the present invention is to provide an improved trenching machine of the type described which is operable to readily and positively grip a conduit lying along a seabed, propel the machine along such conduit at a steady and consistent rate of travel and easily negotiate any obstruction on the conduit without interrupting the travel of the machine.
The present invention provides an improved apparatus for forming a trench in the bed of a body of water for burying a conduit lying therealong which generally consists of a main frame, at least one buoyancy tank mounted on the main frame, a pair of endless track assemblies, each engageable with a side of the conduit being buried, in gripping relation, for propelling the frame along the conduit, and means mounted on the main frame and communicable with a source of fluid under pressure, for ejecting high velocity streams of fluid to dislodge portions of the bed lying below the conduit when the track assemblies grip and propel the main frame of the machine along the conduit to form a trench and allow the conduit to fall therein. Preferably, the fluid injecting means consists of a pair of jet legs mounted on the front end of the machine frame, provided with a plurality of forwardly and laterally directed nozzles for ejecting water under pressure for dislodging material from the seabed along the path of the conduit, and there further is provided a pair of eductors mounted at the rear end of the machine frame, having inlets for drawing spoils produced by the jet legs and ejecting such spoils laterally as the machine moves forwardly.